Stability of Filled PDMS Pervaporation Membranes in Bio-Ethanol Recovery from a Real Fermentation Broth

Mixed matrix membranes (MMMs) have shown great potential in pervaporation (PV). As for many novel membrane materials however, lab-scale testing often involves synthetic feed solutions composed of mixed pure components, overlooking the possibly complex interactions and effects caused by the numerous other components in a real PV feed. This work studies the performance of MMMs with two different types of fillers, a core-shell material consisting of ZIF-8 coated on mesoporous silica and a hollow sphere of silicalite-1, in the PV of a real fermented wheat/hay straw hydrolysate broth for the production of bio-ethanol. All membranes, including a reference unfilled PDMS, show a declining permeability over time. Interestingly, the unfilled PDMS membrane maintains a stable separation factor, whereas the filled PDMS membranes rapidly lose selectivity to levels below that of the reference PDMS membrane. A membrane autopsy using XRD and SEM-EDX revealed an almost complete degradation of the crystalline ZIF-8 in the MMMs. Reference experiments with ZIF-8 nanoparticles in the fermentation broth demonstrated the influence of the broth on the ZIF-8 particles. However, the observed effects from the membrane autopsy could not exactly be replicated, likely due to distinct differences in conditions between the in-situ pervaporation process and the ex-situ reference experiments. These findings raise significant questions regarding the potential applicability of MOF-filled MMMs in real-feed pervaporation processes and, potentially, in harsh condition membrane separations in general. This study clearly confirms the importance of testing membranes in realistic conditions.

Lactobacilli Have a Niche in the Human Nose

Although an increasing number of beneficial microbiome members are characterized for the human gut and vagina, beneficial microbes are underexplored for the human upper respiratory tract (URT). In this study, we demonstrate that taxa from the beneficial Lactobacillus genus complex are more prevalent in the healthy URT than in patients with chronic rhinosinusitis (CRS). Several URT-specific isolates are cultured, characterized, and further explored for their genetic and functional properties related to adaptation to the URT. Catalase genes are found in the identified lactobacilli, which is a unique feature within this mostly facultative anaerobic genus. Moreover, one of our isolated strains, Lactobacillus casei AMBR2, contains fimbriae that enable strong adherence to URT epithelium, inhibit the growth and virulence of several URT pathogens, and successfully colonize nasal epithelium of healthy volunteers. This study thus demonstrates that specific lactobacilli are adapted to the URT and could have a beneficial keystone function in this habitat.

Occurrence and characterisation of biofilms in drinking water systems of broiler houses

Background

Water quality in the drinking water system (DWS) plays an important role in the general health and performance of broiler chickens. Conditions in the DWS of broilers are ideal for microbial biofilm formation. Since pathogens might reside within these biofilms, they serve as potential source of waterborne transmission of pathogens to livestock and humans. Knowledge about the presence, importance and composition of biofilms in the DWS of broilers is largely missing. In this study, we therefore aim to monitor the occurrence, and chemically and microbiologically characterise biofilms in the DWS of five broiler farms.

Results

The bacterial load after disinfection in DWSs was assessed by sampling with a flocked swab followed by enumerations of total aerobic flora (TAC) and Pseudomonas spp. The dominant flora was identified and their biofilm-forming capacity was evaluated. Also, proteins, carbohydrates and uronic acids were quantified to analyse the presence of extracellular polymeric substances of biofilms. Despite disinfection of the water and the DWS, average TAC was 6.03 ± 1.53 log CFU/20cm². Enumerations for Pseudomonas spp. were on average 0.88 log CFU/20cm² lower. The most identified dominant species from TAC were Stenotrophomonas maltophilia, Pseudomonas geniculata and Pseudomonas aeruginosa. However at species level, most of the identified microorganisms were farm specific. Almost all the isolates belonging to the three most abundant species were strong biofilm producers. Overall, 92% of all tested microorganisms were able to form biofilm under lab conditions. Furthermore, 63% of the DWS surfaces appeared to be contaminated with microorganisms combined with at least one of the analysed chemical components, which is indicative for the presence of biofilm.

Conclusions

Stenotrophomonas maltophilia, Pseudomonas geniculata and Pseudomonas aeruginosa are considered as opportunistic pathogens and could consequently be a potential risk for animal health. Additionally, the biofilm-forming capacity of these organisms could promote attachment of other pathogens such as Campylobacter spp. and Salmonella spp.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1451-5) contains supplementary material, which is available to authorized users.

Identification and Spoilage Potential of the Remaining Dominant Microbiota on Food Contact Surfaces after Cleaning and Disinfection in Different Food Industries

After cleaning and disinfection (C&D), surface contamination can still be present in the production environment of food companies. Microbiological contamination on cleaned surfaces can be transferred to the manufactured food and consequently lead to foodborne illness and early food spoilage. However, knowledge about the microbiological composition of residual contamination after C&D and the effect of this contamination on food spoilage is lacking in various food sectors. In this study, we identified the remaining dominant microbiota on food contact surfaces after C&D in seven food companies and assessed the spoilage potential of the microbiota under laboratory conditions. The dominant microbiota on surfaces contaminated at ≥10² CFU/100 cm² after C&D was identified based on 16S rRNA sequences. The ability of these microorganisms to hydrolyze proteins, lipids, and phospholipids, ferment glucose and lactose, produce hydrogen sulfide, and degrade starch and gelatin also was evaluated. Genera that were most abundant among the dominant microbiota on food contact surfaces after C&D were Pseudomonas, Microbacterium, Stenotrophomonas, Staphylococcus, and Streptococcus. Pseudomonas spp. were identified in five of the participating food companies, and 86.8% of the isolates evaluated had spoilage potential in the laboratory tests. Microbacterium and Stenotrophomonas spp. were identified in five and six of the food companies, respectively, and all tested isolates had spoilage potential. This information will be useful for food companies in their quest to characterize surface contamination after C&D, to identify causes of microbiological food contamination and spoilage, and to determine the need for more thorough C&D.

Evaluation of Two Surface Sampling Methods for Microbiological and Chemical Analyses To Assess the Presence of Biofilms in Food Companies

Biofilms are an important source of contamination in food companies, yet the composition of biofilms in practice is still mostly unknown. The chemical and microbiological characterization of surface samples taken after cleaning and disinfection is very important to distinguish free-living bacteria from the attached bacteria in biofilms. In this study, sampling methods that are potentially useful for both chemical and microbiological analyses of surface samples were evaluated. In the manufacturing facilities of eight Belgian food companies, surfaces were sampled after cleaning and disinfection using two sampling methods: the scraper-flocked swab method and the sponge stick method. Microbiological and chemical analyses were performed on these samples to evaluate the suitability of the sampling methods for the quantification of extracellular polymeric substance components and microorganisms originating from biofilms in these facilities. The scraper-flocked swab method was most suitable for chemical analyses of the samples because the material in these swabs did not interfere with determination of the chemical components. For microbiological enumerations, the sponge stick method was slightly but not significantly more effective than the scraper-flocked swab method. In all but one of the facilities, at least 20% of the sampled surfaces had more than 10² CFU/100 cm². Proteins were found in 20% of the chemically analyzed surface samples, and carbohydrates and uronic acids were found in 15 and 8% of the samples, respectively. When chemical and microbiological results were combined, 17% of the sampled surfaces were contaminated with both microorganisms and at least one of the analyzed chemical components; thus, these surfaces were characterized as carrying biofilm. Overall, microbiological contamination in the food industry is highly variable by food sector and even within a facility at various sampling points and sampling times.

Fed-batch production of green coconut hydrolysates for high-gravity second-generation bioethanol fermentation with cellulosic yeast

The residual biomass obtained from the production of Cocos nucifera L. (coconut) is a potential source of feedstock for bioethanol production. Even though coconut hydrolysates for ethanol production have previously been obtained, high-solid loads to obtain high sugar and ethanol levels remain a challenge. We investigated the use of a fed-batch regime in the production of sugar-rich hydrolysates from the green coconut fruit and its mesocarp. Fermentation of the hydrolysates obtained from green coconut or its mesocarp, containing 8.4 and 9.7% (w/v) sugar, resulted in 3.8 and 4.3% (v/v) ethanol, respectively. However, green coconut hydrolysate showed a prolonged fermentation lag phase. The inhibitor profile suggested that fatty acids and acetic acid were the main fermentation inhibitors. Therefore, a fed-batch regime with mild alkaline pretreatment followed by saccharification, is presented as a strategy for fermentation of such challenging biomass hydrolysates, even though further improvement of yeast inhibitor tolerance is also needed.

Green coconut mesocarp pretreated by an alkaline process as raw material for bioethanol production

Cocos nucifera L., coconut, is a palm of high importance in the food industry, but a considerable part of the biomass is inedible. In this study, the pretreatment and saccharification parameters NaOH solution, pretreatment duration and enzyme load were evaluated for the production of hydrolysates from green coconut mesocarp using 18% (w/v) total solids (TS). Hydrolysates were not detoxified in order to preserve sugars solubilized during the pretreatment. Reduction of enzyme load from 15 to 7.5 filter paper cellulase unit (FPU)/g of biomass has little effect on the final ethanol titer. With optimized pretreatment and saccharification, hydrolysates with more than 7% (w/v) sugars were produced in 48h. Fermentation of the hydrolysate using industrial Saccharomyces cerevisiae strains produced 3.73% (v/v) ethanol. Our results showed a simple pretreatment condition with a high-solid load of biomass followed by saccharification and fermentation of undetoxified coconut mesocarp hydrolysates to produce ethanol with high titer.

Large-scale robot-assisted genome shuffling yields industrial Saccharomyces cerevisiae yeasts with increased ethanol tolerance

BACKGROUND

During the final phases of bioethanol fermentation, yeast cells face high ethanol concentrations. This stress results in slower or arrested fermentations and limits ethanol production. Novel Saccharomyces cerevisiae strains with superior ethanol tolerance may therefore allow increased yield and efficiency. Genome shuffling has emerged as a powerful approach to rapidly enhance complex traits including ethanol tolerance, yet previous efforts have mostly relied on a mutagenized pool of a single strain, which can potentially limit the effectiveness. Here, we explore novel robot-assisted strategies that allow to shuffle the genomes of multiple parental yeasts on an unprecedented scale.

RESULTS

Screening of 318 different yeasts for ethanol accumulation, sporulation efficiency, and genetic relatedness yielded eight heterothallic strains that served as parents for genome shuffling. In a first approach, the parental strains were subjected to multiple consecutive rounds of random genome shuffling with different selection methods, yielding several hybrids that showed increased ethanol tolerance. Interestingly, on average, hybrids from the first generation (F1) showed higher ethanol production than hybrids from the third generation (F3). In a second approach, we applied several successive rounds of robot-assisted targeted genome shuffling, yielding more than 3,000 targeted crosses. Hybrids selected for ethanol tolerance showed increased ethanol tolerance and production as compared to unselected hybrids, and F1 hybrids were on average superior to F3 hybrids. In total, 135 individual F1 and F3 hybrids were tested in small-scale very high gravity fermentations. Eight hybrids demonstrated superior fermentation performance over the commercial biofuel strain Ethanol Red, showing a 2 to 7% increase in maximal ethanol accumulation. In an 8-l pilot-scale test, the best-performing hybrid fermented medium containing 32% (w/v) glucose to dryness, yielding 18.7% (v/v) ethanol with a productivity of 0.90 g ethanol/l/h and a yield of 0.45 g ethanol/g glucose.

CONCLUSIONS

We report the use of several different large-scale genome shuffling strategies to obtain novel hybrids with increased ethanol tolerance and fermentation capacity. Several of the novel hybrids show best-parent heterosis and outperform the commonly used bioethanol strain Ethanol Red, making them interesting candidate strains for industrial production.

Phenotypic evaluation of natural and industrial Saccharomyces yeasts for different traits desirable in industrial bioethanol production

Saccharomyces cerevisiae is the organism of choice for many food and beverage fermentations because it thrives in high-sugar and high-ethanol conditions. However, the conditions encountered in bioethanol fermentation pose specific challenges, including extremely high sugar and ethanol concentrations, high temperature, and the presence of specific toxic compounds. It is generally considered that exploring the natural biodiversity of Saccharomyces strains may be an interesting route to find superior bioethanol strains and may also improve our understanding of the challenges faced by yeast cells during bioethanol fermentation. In this study, we phenotypically evaluated a large collection of diverse Saccharomyces strains on six selective traits relevant for bioethanol production with increasing stress intensity. Our results demonstrate a remarkably large phenotypic diversity among different Saccharomyces species and among S. cerevisiae strains from different origins. Currently applied bioethanol strains showed a high tolerance to many of these relevant traits, but several other natural and industrial S. cerevisiae strains outcompeted the bioethanol strains for specific traits. These multitolerant strains performed well in fermentation experiments mimicking industrial bioethanol production. Together, our results illustrate the potential of phenotyping the natural biodiversity of yeasts to find superior industrial strains that may be used in bioethanol production or can be used as a basis for further strain improvement through genetic engineering, experimental evolution, or breeding. Additionally, our study provides a basis for new insights into the relationships between tolerance to different stressors.

Development of a D-xylose fermenting and inhibitor tolerant industrial Saccharomyces cerevisiae strain with high performance in lignocellulose hydrolysates using metabolic and evolutionary engineering

BACKGROUND

The production of bioethanol from lignocellulose hydrolysates requires a robust, D-xylose-fermenting and inhibitor-tolerant microorganism as catalyst. The purpose of the present work was to develop such a strain from a prime industrial yeast strain, Ethanol Red, used for bioethanol production.

RESULTS

An expression cassette containing 13 genes including Clostridium phytofermentans XylA, encoding D-xylose isomerase (XI), and enzymes of the pentose phosphate pathway was inserted in two copies in the genome of Ethanol Red. Subsequent EMS mutagenesis, genome shuffling and selection in D-xylose-enriched lignocellulose hydrolysate, followed by multiple rounds of evolutionary engineering in complex medium with D-xylose, gradually established efficient D-xylose fermentation. The best-performing strain, GS1.11-26, showed a maximum specific D-xylose consumption rate of 1.1 g/g DW/h in synthetic medium, with complete attenuation of 35 g/L D-xylose in about 17 h. In separate hydrolysis and fermentation of lignocellulose hydrolysates of Arundo donax (giant reed), spruce and a wheat straw/hay mixture, the maximum specific D-xylose consumption rate was 0.36, 0.23 and 1.1 g/g DW inoculum/h, and the final ethanol titer was 4.2, 3.9 and 5.8% (v/v), respectively. In simultaneous saccharification and fermentation of Arundo hydrolysate, GS1.11-26 produced 32% more ethanol than the parent strain Ethanol Red, due to efficient D-xylose utilization. The high D-xylose fermentation capacity was stable after extended growth in glucose. Cell extracts of strain GS1.11-26 displayed 17-fold higher XI activity compared to the parent strain, but overexpression of XI alone was not enough to establish D-xylose fermentation. The high D-xylose consumption rate was due to synergistic interaction between the high XI activity and one or more mutations in the genome. The GS1.11-26 had a partial respiratory defect causing a reduced aerobic growth rate.

CONCLUSIONS

An industrial yeast strain for bioethanol production with lignocellulose hydrolysates has been developed in the genetic background of a strain widely used for commercial bioethanol production. The strain uses glucose and D-xylose with high consumption rates and partial cofermentation in various lignocellulose hydrolysates with very high ethanol yield. The GS1.11-26 strain shows highly promising potential for further development of an all-round robust yeast strain for efficient fermentation of various lignocellulose hydrolysates.